Altimeters are used to determine an altitude above a reference level, typically the mean sea level or the standard sea level. One type of altimeter determines altitude based on a measurement of atmospheric pressure. This type of altimeter is referred to as a pressure altimeter or a barometric altimeter. Atmospheric pressure is a suitable medium for determining altitude because, as altitude increases the atmospheric pressure decreases at a known rate.
Atmospheric pressure is influenced by factors including weather fronts and temperature; accordingly, before each use pressure altimeters are calibrated or aligned to a reference pressure that corresponds to a known altitude. For example, a pressure altimeter in an aircraft in flight is regularly adjusted to local sea level pressure in order to achieve a suitably accurate altitude reading.
When using a pressure altimeter to determine altitude as part of an indoor navigation system, the pressure altimeter should be calibrated to a trusted reference pressure and an associated known altitude that is as close as possible to the navigation site. Weather stations, schools, and airports are typically used as sources of trusted and reliable reference pressures and known altitudes. Problematically, these sources of trusted data are often far enough from the indoor navigation site that the atmospheric pressure at the navigation site is different from the atmospheric pressure at the source site. Additionally, in some situations the atmospheric pressure is different outside of a building as compared to the pressure inside of the building. Often, the difference in pressure is caused by heating, ventilation, and cooling equipment (“HVAC”) associated with the building, for example. Accordingly, the differences in atmospheric pressure described above, typically result in an improperly calibrated pressure altimeter that generates inaccurate indoor navigation results.
Therefore, it is desirable to provide a method and a system for determining a suitable reference pressure and associated known altitude to calibrate a pressure altimeter to be used with an indoor navigation system.